The present invention relates particularly, although by no means exclusively, to a method of starting a molten bath-based smelting process for producing molten metal from a metalliferous feed material in a smelting vessel that has a strong bath/slag fountain generated by gas evolution in the molten bath, with the gas evolution being at least partly the result of devolatilisation of carbonaceous material in the molten bath.
In particular, although by no means exclusively, the present invention relates to a method of starting a process for smelting an iron-containing material, such as an iron ore, and producing iron.
The present invention relates particularly, although by no means exclusively, to a method of starting a smelting process in a smelting vessel that includes a main chamber for smelting metalliferous material.
A known molten bath-based smelting process, generally referred to as the HIsmelt process, is described in a considerable number of patents and patent applications in the name of the applicant.
Another molten bath-based smelting process, referred to hereinafter as the “HIsarna” process, is described in International application PCT/AU99/00884 (WO 00/022176) in the name of the applicant.
The HIsmelt process and the HIsarna process are associated particularly with producing molten iron from iron ore or another iron-containing material.
The HIsarna process is carried out in a smelting apparatus that includes (a) a smelting vessel that includes a main smelting chamber and lances for injecting solid feed materials and oxygen-containing gas into the main chamber and is adapted to contain a bath of molten metal and slag and (b) a smelt cyclone for pre-treating a metalliferous feed material that is positioned above and communicates directly with the smelting vessel.
The term “smelt cyclone” is understood herein to mean a vessel that typically defines a vertical cylindrical chamber and is constructed so that feed materials supplied to the chamber move in a path around a vertical central axis of the chamber and can withstand high operating temperatures sufficient to at least partially melt metalliferous feed materials.
In one form of the HIsarna process, carbonaceous feed material (typically coal) and optionally flux (typically calcined limestone) are injected into a molten bath in the main chamber of the smelting vessel. The carbonaceous material is provided as a source of a reductant and a source of energy. Metalliferous feed material, such as iron ore, optionally blended with flux, is injected into and heated and partially melted and partially reduced in the smelt cyclone. This molten, partly reduced metalliferous material flows downwardly from the smelt cyclone into the molten bath in the smelting vessel and is smelted to molten metal in the bath. Hot reaction gases (typically CO, CO2, H2, and H2O) produced in the molten bath is partially combusted by oxygen-containing gas (typically technical-grade oxygen) in an upper part of the main chamber. Heat generated by the post-combustion is transferred to molten droplets in the upper section that fall back into the molten bath to maintain the temperature of the bath. The hot, partially-combusted reaction gases flow upwardly from the main chamber and enter the bottom of the smelt cyclone. Oxygen-containing gas (typically technical-grade oxygen) is injected into the smelt cyclone via tuyeres that are arranged in such a way as to generate a cyclonic swirl pattern in a horizontal plane, i.e. about a vertical central axis of the chamber of the smelt cyclone. This injection of oxygen-containing gas leads to further combustion of smelting vessel gases, resulting in very hot (cyclonic) flames. Finely divided incoming metalliferous feed material is injected pneumatically into these flames via tuyeres in the smelt cyclone, resulting in rapid heating and partial melting accompanied by partial reduction (roughly 10-20% reduction). The reduction is due to both thermal decomposition of hematite and the reducing action of CO/H2 in the reaction gases from the main chamber. The hot, partially melted metalliferous feed material is thrown outwards onto the walls of the smelt cyclone by cyclonic swirl action and, as described above, flows downwardly into the smelting vessel below for smelting in the main chamber of that vessel.
The net effect of the above-described form of the HIsarna process is a two-step countercurrent process. Metalliferous feed material is heated and partially reduced by outgoing reaction gases form the smelting vessel (with oxygen-containing gas addition) and flows downwardly into the smelting vessel and is smelted to molten iron in the smelting vessel. In a general sense, this countercurrent arrangement increases productivity and energy efficiency.
The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.
The applicant has proposed that the HIsarna process and an oxygen-blown version of the HIsmelt process be started up in a smelting vessel by feeding hot metal (from an external source) into the main chamber of the vessel via the forehearth of the vessel, commencing supplying oxygen-containing gas (typically technical grade oxygen) and solid carbonaceous material (typically coal) and generating heat in the main chamber. This hot start-up method generates heat via spontaneous ignition of combustible material in the main chamber. The applicant has proposed that this initial step in the hot start-up method be followed by the addition of slag-forming agents and, later on, by the addition of metalliferous feed material (such as ferruginous material such as iron ore) into the main chamber. The hot start method is described in a companion International application entitled “Starting a Smelting Process” lodged in the name of the applicant on the same day as the subject International application for the present invention.
In pilot plant trials of the HIsarna process that were based on technical grade oxygen as the oxygen-containing gas, coal as the solid carbonaceous material, and iron ore fines as the metalliferous material, the above hot start-up method was tested. The applicant found that there is a finite time-window after pouring hot metal into the main chamber of the smelting vessel within which it is possible to safely commence supplying cold oxygen and coal into the main chamber and have spontaneous ignition of combustible material and generate heat in the main chamber which is necessary to start up the process. This time window was found to be typically around 1-2 hours in duration under the pilot plant conditions. The time-window was found to be variable depending on (amongst other factors) smelting vessel geometry, charge metal temperature and charge metal chemistry. It was also found that if the steps of commencing supplying oxygen and coal into the main chamber were not implemented within the requisite time-window, it became impossible to guarantee spontaneous ignition of coal and cold oxygen inside the main chamber. This resulted in a mixture of un-combusted coal and oxygen leaving the main chamber, with a possibility of a coal dust explosion (and associated damage) in downstream equipment.
The applicant has found that the mechanism associated with this time-window is related to the formation of a slag layer on top of the molten metal. Once this slag layer is sufficiently well established, it is cooled by radiation to the smelting vessel, for example to water panels in a top space of the main chamber, and becomes crusty, effectively creating a blanket on the molten metal. The blanket acts as a thermal insulator that restricts heat transfer from the molten metal below the blanket to the top space of the main chamber above the blanket, with a result that the thermal conditions in the top space are too cold to support spontaneous ignition of combustible material in the top space. Whilst not wishing to be bound by the following comment, the applicant believes that this slag formed during the pilot plant trials primarily from (i) residual slag coatings left behind from previous operations and (ii) oxygen reacting with constituents in molten metal (silicon in particular, reacting to form silicon dioxide).
The applicant also found in the pilot plant trials that essentially the same problem occurred if the plant was operating normally and was then stopped (including stopping supplying solid feed materials to the main chamber) for a significant period of time (for example, to perform mechanical repairs outside the smelting vessel). Under these conditions the slag layer is generally significantly thicker compared to the slag layer at start-up of the process. Hot metal is significantly further below the surface of the slag, and is therefore less able to keep the slag upper surface hot by conduction. Surface slag heat was lost to the smelting vessel, for example to water-cooled panels in the top space of the main chamber, by radiation, and a cold, crusty layer formed on the molten bath more rapidly than before. When the smelting vessel was idle for more than about 15-30 minutes, spontaneous ignition of coal and cold oxygen and hence heat generation required to support process start-up in the main chamber of the smelting vessel was once again impossible to guarantee.
During the course of the pilot plant trials the applicant developed a safe, practical method to start the HIsarna process under these conditions. An important basis of the method is recognising the necessity (before cold oxygen and coal or other suitable oxygen-containing gas and solid carbonaceous material can be introduced) to establish a sufficiently large and stable “hot zone” for ignition of oxygen and coal in the main chamber of the smelting vessel by independent means, i.e. independently of and before supplying cold oxygen and coal into the main chamber.
The method of starting a smelting process of the present invention is applicable to starting (which term includes “re-starting”) any molten bath-based smelting process when the thermal conditions (temperatures) inside a top space of the main chamber of the smelting vessel are too cold to allow safe, spontaneous ignition of any newly supplied cold oxygen-containing gas and solid carbonaceous material into the main chamber. As described above, such conditions are typically encountered when a crusty frozen slag layer is present on the upper surface of the molten bath in the main chamber.